The mAP values of the detector when it is evaluated on closed set data (trained and tested on Pascal VOC 2007) and open set data (test set contains equal number of unknown images from MS-COCO) helps to measure how the detector handles unknown instances. Ideally, there should not be a performance drop. We compare ORE against the recent open set detector proposed by Miller [1]}. We find from Tab. REF  that drop in performance of ORE is much lower than [1]} owing to the effective modelling of the unknown instances.
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